Same principles apply as to horses. 2% of their body weight in forage as a starting point - all they want to eat if they need the calories, some restriction if they’re a porker. Keep the body condition a healthy lean. But never sacrifice nutrition for calories.
Ration balancers work well for pony foals as well as horse foals. Triple Crown Senior is a great growth feed as well, nice low NSC. But a ration balancer as a base, with some alfalfa pellets for calories as needed (you can keep a handful in the diet all the time, then increase/decrease as calories are needed), is a great way to keep up with the changing body condition of fast-growing youngsters, without changing back and forth between a growth feed and balancer.